Roles of HynAB and Ech, the Only Two Hydrogenases Found in the Model Sulfate Reducer <i>Desulfovibrio gigas</i>

Abstract

Sulfate-reducing bacteria are characterized by a high number of hydrogenases, which have been proposed to contribute to the overall energy metabolism of the cell, but exactly in what role is not clear. Desulfovibrio spp. can produce or consume H2 when growing on organic or inorganic substrates in the presence or absence of sulfate. Because of the presence of only two hydrogenases encoded in its genome, the periplasmic HynAB and cytoplasmic Ech hydrogenases, Desulfovibrio gigas is an excellent model organism for investigation of the specific function of each of these enzymes during growth. In this study, we analyzed the physiological response to the deletion of the genes that encode the two hydrogenases in D. gigas, through the generation of ΔechBC and ΔhynAB single mutant strains. These strains were analyzed for the ability to grow on different substrates, such as lactate, pyruvate, and hydrogen, under respiratory and fermentative conditions. Furthermore, the expression of both hydrogenase genes in the three strains studied was assessed through quantitative reverse transcription-PCR. The results demonstrate that neither hydrogenase is essential for growth on lactate-sulfate, indicating that hydrogen cycling is not indispensable. In addition, the periplasmic HynAB enzyme has a bifunctional activity and is required for growth on H2 or by fermentation of pyruvate. Therefore, this enzyme seems to play a dominant role in D. gigas hydrogen metabolism.